1.Technical Field
The present invention relates to an electro-hydraulic actuation system for use in a hydraulic shovel or asphalt finisher and more particularly to an electro-hydraulic actuation system to which a load sensing system (hereinafter, referred to as LS) is applied for changing the discharge volume of a pump based on the load pressure of an actuator so that a differential pressure between the load pressure of the actuator and the discharge pressure of the pump becomes substantially constant.
2. Background Art
Conventionally, as a hydraulic actuation system to which the LS is applied, there has been known, for example, a hydraulic actuation system 900 shown in FIG. 22.
The hydraulic actuation system 900 includes a variable displacement hydraulic pump 911 for discharging a working fluid, a tank 912 from which the working fluid is discharged and a relief valve 913 for keeping the working fluid discharge pressure of the variable displacement hydraulic pump 911 at a predetermined set pressure or lower.
In addition, the hydraulic actuation system 900 includes a unit 920 having reduction gears 921 connected to a load, not shown, a variable displacement hydraulic motor 922 for imparting a driving force to the reduction gears 921, an operation lever 923 adapted to be operated by an operator, a manual direction control valve 924 for changing the communication of the variable displacement hydraulic pump 911 and the tank 912 with the variable displacement hydraulic motor 922 in accordance with the operation amount of the operation lever 923 and an automatic two-position valve 925 for cutting off the communication of the variable displacement hydraulic pump 911 and the tank 912 with the variable displacement hydraulic motor 922 in accordance with the operation amount of the operation lever 923 when the communication of the variable displacement hydraulic pump 911 and the tank 912 with the variable displacement hydraulic motor 922 in accordance with the operation amount of the operation lever 923 is cut off by the manual direction control valve 924.
In addition, the hydraulic actuation system 900 includes a unit 930 having a similar configuration to that of the unit 920, while the detailed description of the unit 930 is omitted here.
Additionally, the hydraulic actuation system 900 includes a selector valve 941 for selecting a working fluid having a greater pressure of a working fluid supplied from the variable displacement hydraulic pump 911 to the variable displacement hydraulic motor 922 of the unit 920 and a working fluid supplied to the variable displacement hydraulic pump 911 to a variable displacement hydraulic motor 932 of the unit 930, a two-position electromagnetic valve 942 for allowing the working fluid discharged by the variable displacement hydraulic pump 911 to pass when a pressure resulting from adding a predetermined set pressure to the pressure of the working fluid selected by the selector valve 941 is greater than the working fluid discharge pressure of the variable displacement hydraulic pump 911 discharges the working fluid, a discharge volume changing hydraulic cylinder 943 for changing the discharge volume of the variable displacement hydraulic pump 911 based on the pressure of the working fluid which is allowed to pass by the two-position electromagnetic valve 942 and the working fluid discharge pressure of the variable displacement hydraulic pump 911.
By the configuration that has been described above, the discharge volume changing hydraulic cylinder 943 used to change the working fluid discharge volume of the variable displacement hydraulic pump 911 so that a difference between a larger pressure of the pressure of the working fluid supplied from the variable displacement hydraulic pump 911 to the variable displacement hydraulic motor 922 of the unit 920, that is, the load pressure of the unit 920 and the pressure of the working fluid supplied from the variable displacement hydraulic pump 911 to the variable displacement hydraulic motor 932 of the unit 930, that is, the load pressure of the unit 930 and the working fluid discharge pressure of the variable displacement hydraulic pump 911 becomes the set pressure of the two-position electromagnetic valve 942.
In the conventional hydraulic actuation system 900, however, there was a problem that the amount of the working fluid is short which is supplied to the variable displacement hydraulic motor having a larger load pressure of the variable displacement hydraulic motor 922 of the unit 920 and the variable displacement hydraulic motor 932 of the unit 930.
When the load pressure of the variable displacement hydraulic motor 922 of the unit 920 or the variable displacement hydraulic motor 932 of the unit 930 is increased, the working fluid discharge pressure of the variable displacement hydraulic pump 911 is increased due to the actions of the selector valve 941, the two-position electromagnetic valve 942 and the discharge volume changing hydraulic cylinder 943. However, in a case where the relationship between the working fluid discharge volume and discharge pressure of the variable displacement hydraulic pump 911 is such as shown in FIG. 23, namely, in a case where the variable displacement hydraulic pump 911 is a pump whose horse power is constant, the working fluid discharge volume of the variable displacement hydraulic pump 911 decreases as the working fluid discharge pressure thereof increases.
Here, when the working fluid discharge volume of the variable displacement hydraulic pump 911 becomes smaller than a total amount of working fluid needed to be supplied to the variable displacement hydraulic motor 922 of the unit 920 and the variable displacement hydraulic motor 932 of the unit 930, the working fluid discharged by the variable displacement hydraulic pump 911 flows to the variable displacement hydraulic motor having a smaller load pressure in preference to the other.
Then, there occurs a shortage in volume of the working fluid supplied to the variable displacement hydraulic motor having a larger load pressure of the variable displacement hydraulic motor 922 of the unit 920 and the variable displacement hydraulic motor 932 of the unit 930, and the output thereof is reduced when compared with a case where there occurs no shortage in volume of the working fluid supplied thereto.
Consequently, for example, in a case where the variable displacement hydraulic motor 922 of the unit 920 is used for driving a right side caterpillar of a hydraulic shovel and the variable displacement hydraulic motor 932 of the unit 930 is used for a left side caterpillar of the hydraulic shovel, when the operator attempts to move the hydraulic shovel straight forward by inputting substantially equal operation amounts to the operation lever 923 of the unit 920 and an operation lever 933 of the unit 930, in the event that a load borne by the right side caterpillar becomes larger than a load borne by the left side caterpillar as a result of, for example, the right side caterpillar riding on a stone or the left side caterpillar entering a puddle, the movement of the right side caterpillar becomes slower than the movement of the left side caterpillar, and the hydraulic shovel advances while turning to the right.
Then, an object of the invention is to provide an electro-hydraulic actuation system which can prevent the generation of a shortage in volume of a fluid supplied to a hydraulic actuator having a larger load pressure of hydraulic actuators of a plurality of electro-hydraulic actuators.
3. Disclosure of the Invention
With a view to solving the problem, according to the invention, there is provided an electro-hydraulic actuation system including a pump, a plurality of electro-hydraulic actuators each having an electric motor, a hydraulic actuator and a fluid volume changing valve for changing the volume of a fluid discharged by the pump based on driving amounts of the electric motor and the hydraulic actuator for supply to the hydraulic actuator, a discharge volume changing means for changing the volume of the fluid discharged by the pump based on a maximum pressure of pressures of the fluid supplied to the hydraulic actuators of the plurality of electro-hydraulic actuators and the discharge pressure of the fluid discharged by the pump and a rotational speed changing means for changing the rotational speed of the electric motors of the plurality of electro-hydraulic actuators at substantially the same ratio relative to the electric motors of the plurality of electro-hydraulic actuators based on a maximum pressure of pressures of the fluid supplied to the hydraulic actuators of the plurality of electro-hydraulic actuators and the discharge pressure of the fluid discharged by the pump.
By adopting this configuration, since the electro-hydraulic actuation system of the invention can reduce the volume of the fluid supplied to the hydraulic actuators of the plurality of electro-hydraulic actuators at substantially the same ratio when there occurs a shortage in volume of the fluid supplied to the hydraulic actuators of the plurality of electro-hydraulic actuators, it is possible to prevent the occurrence of a shortage in volume of the fluid supplied to the actuator having a larger load pressure of the hydraulic actuators of the plurality of electro-hydraulic actuators.